Published online by Cambridge University Press: 11 August 2009
Introduction
Genetic diversity is the information that is contained in the genes of individual plants, animals and micro-organisms. Species diversity is the diversity of species within which gene flow occurs under natural conditions. Agricultural biodiversity is defined as a component of biodiversity, referring to all diversity within and among species found in crop and domesticated livestock systems, including wild relatives, interacting species of pollinators, pests, parasites and other organisms (Qualset et al. 1995; Wood and Lenné 1999; Smale and Drucker 2007).
In managed systems, such as agro-ecosystems, crop genetic resources are the raw materials for modern crop breeding, selection programmes, pest resistance, productivity, stability and future agronomic improvements. A number of studies in the agro-ecology literature suggest that genetic variability within and between crop species confers the potential to resist stress, provide shelter from adverse conditions and increase the resilience and sustainability of agro-ecosystems. Crop biodiversity erosion increases the vulnerability of the crop to biotic and abiotic stresses. Biodiversity reduction promotes build-up of crop pest and pathogen populations. Plot studies show that intercropping would reduce the probability of absolute failure of crop and that crop diversification increases crop income stability (Walker et al. 1983).
Therefore, the greater the diversity between and/or within species and functional groups, the greater the tolerance to pests. This is because pests have more ability to spread through crops with the same genetic base (Sumner et al. 1981; Altieri and Lieberman 1986; Gliessman 1986; Heisey et al. 1997).
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